FIG. 1 shows a diagram illustrating an exemplary multi-channel digital printing system 10 for printing on a web of receiver medium 14. The printing system 10 includes a plurality of printing modules 12, each adapted to print image data for an image plane corresponding to a different color channel. In some printing systems 10, the printing modules 12 are inkjet printing modules adapted to print drops of ink onto the receiver medium 14 through an array of inkjet nozzles. In other cases, the printing modules 12 can be electrophotographic printing modules that produce images by applying solid or liquid toner to the receiver medium 14. Alternately, the printing modules 12 can utilize any type of digital printing technology known in the art.
In the illustrated example, the printing modules 12 print cyan (C), magenta (M), yellow (Y) and black (K) colorants (e.g., inks) onto the receiver medium 14 as it is transported through the printing system using a media transport system (not shown in FIG. 1) from an upstream to a downstream in a receiver motion direction 16. (The receiver medium direction 16 is commonly referred to as the “in-track direction,” and the direction perpendicular to the receiver medium direction 16 is commonly referred to as the “cross-track direction.”) In other cases, the printing modules 12 can be adapted to print different numbers and types of colorants. For example, additional printing modules 12 can be used to print specialty colorants, or extended gamut colorants. In some cases, a plurality of the printing modules 12 can be used to print the same colorant (e.g., black), or density variations of the same color (e.g., gray and black). In some cases, the printing system 10 is adapted to print double-sided pages. In this case, one or more of the printing modules 12 can be arranged to print on a back side of the receiver medium 14.
The printing system 10 also includes dryers 18 for drying the ink applied to the receiver medium 14 by the printing modules. While the exemplary printing system 10 illustrates a dryer 18 following each of the printing modules 12, this is not a requirement. In some cases, a single dryer 18 may be used following the last printing module 12, or dryers 18 may only be provided following some subset of the printing modules 12. Depending on the printing technology used in the printing modules 12, and the printing speed, it may not be necessary to use any dryers 18.
Downstream of the printing modules 12, an imaging system 20, which can include one or more imaging devices 22 is used for capturing images of printed images on the receiver medium 14. In some cases, the imaging system 20 can include a single imaging device 22 that captures an image of the entire width of the receiver medium 14, or of a relevant portion thereof. In other cases, a plurality of imaging devices 22 can be used, each of which captures an image of a corresponding portion of the printed image. In some embodiments, the position of the imaging devices 22 can be adjusted during a calibration process to sequentially capture images of different portions of the receiver medium 14. For cases where the printing system 10 prints double-sided images, some of the imaging devices 22 may be adapted to capture images of a second side of the receiver medium 14.
In some cases, the imaging devices 22 can be digital camera systems adapted to capture 2-D images of the receiver medium 14. In other embodiments, the imaging devices 22 can include 1-D linear sensors that are used to capture images of the receiver medium 14 on a line-by-line basis as the receiver medium 14 moves past the imaging system 20. The imaging devices 22 can equivalently be referred to as “cameras” or “camera systems” or “scanners” or “scanning systems,” independent of whether they utilize 2-D or 1-D imaging sensors. Similarly, the images provided by the imaging devices 22 can be referred to as “captured images” or “scanned images” or “scans.” In some cases, the imaging devices 22 include color sensors for capturing color images of the receiver medium, to more easily distinguish between the colorants deposited by the different printing modules 12.
FIG. 2 is a diagram of an exemplary printing module 12. In this configuration, the printing module 12 is an inkjet printing system that includes a plurality of inkjet printheads 30 arranged across a width dimension of the receiver medium 14 in a staggered array configuration. (The width dimension of the receiver medium 14 is the dimension perpendicular to the receiver motion direction 16.) Such inkjet printing modules 12 are sometimes referred to as “lineheads.”
Each of the inkjet printheads 30 includes a plurality of inkjet nozzles arranged in nozzle array 31, and is adapted to print a swath of image data in a corresponding printing region 32. In the illustrated example, the nozzle arrays 31 are one-dimensional linear arrays, but the invention is also applicable to inkjet printheads 30 having nozzles arrayed in two-dimensional arrays as well. Common types of inkjet printheads 30 include continuous inkjet (CI) printheads and drop-on-demand (DOD) printheads. Commonly, the inkjet printheads 30 are arranged in a spatially-overlapping arrangement where the printing regions 32 overlap in overlap regions 34. Each of the overlap regions 34 has a corresponding centerline 36. In the overlap regions 34, nozzles from more than one nozzle array 31 can be used to print the image data.
Stitching is a process that refers to the alignment of the printed images produced from multiple printheads 30 for the purpose of creating the appearance of a single page-width line head. For example, as shown in FIG. 2, six printheads 30, each three inches in length, can be stitched together at overlap regions 34 to form an eighteen inch page-width printing module 12. The page-width image data is processed and segmented into separate portions that are sent to each printhead 30 with appropriate time delays to account for the staggered positions of the printheads 30. The image data portions printed by each of the printheads 30 is sometimes referred to as “swaths.” Stitching systems and algorithms are used to determine which nozzles of each nozzle array 31 should be used for printing in the overlap region 34. Preferably, the stitching algorithms create a boundary between the printing regions 32 that is not readily detected by eye. One such stitching algorithm is described in commonly-assigned U.S. Pat. No. 7,871,145 to Enge, entitled “Printing method for reducing stitch error between overlapping jetting modules,” which is incorporated herein by reference.
One problem which is common in printing systems 10 that include a plurality of printheads 30 is alignment of the image data printed by the different printheads 30. There are a variety of different types of alignment errors that can occur. For color printing systems 10 having a plurality of different printing modules 12, the image data printed by one printing module 12 (e.g., a first color channel) can be misaligned with the image data printed by a second printing module 12 (e.g., a second color channel). These color-to-color alignment errors can occur in either or both of the in-track direction or the cross-track direction. Similarly, for printing modules 12 that include a plurality of printheads 30 the image data printed by one printhead 30 can be misaligned with the image data printed by a second printhead 30.
The alignment errors can result from a variety of different causes. In some cases, the alignment can result from variations in the geometry of the printheads 30 during manufacturing, and variations in the positioning of the printheads 30 within the printing system 10. In other cases, alignment errors can result from interactions between the printing system 10 and the environment (e.g., airflow perturbations can cause ink drops to be misdirected in inkjet printing systems). Another common source of misalignment is dimensional changes in the receiver medium 14 that can occur as the receiver medium 14 moves between different printing modules 12. For example, the absorption of water in the ink printed by one channel can cause the receiver medium 14 to expand before a subsequent channel is printed. Similarly, when the receiver medium 14 passes through a dryer, this can cause the receiver medium 14 to shrink. Such dimensional changes in the receiver medium 14 will generally be a function of a variety of factors such as media type, image content of the printed image, and environmental conditions. Dimensional changes can also result from other types of processing operations that are performed between the printing of one channel and another. For example, in an electrophotographic printing system, a fusing operation may be performed between the printing of a front side image and a back side image that can produce dimensional changes of the receiver medium 14.
A variety of different methods have been proposed in the prior art to detect and correct for alignment errors. Typically, the methods involve printing test patterns and capturing an image of the printed test pattern to characterize the alignment errors. Appropriate adjustments can then be made to correct for the alignment errors. In some cases, the adjustments can involve adjusting the physical positions of system components (e.g., the printing modules). In other cases, the adjustments can involve modifying the image data sent to the printheads 30 (e.g., by shifting the image data) or modifying time delays between the time that the image data is printed by one printhead 30 and the time that the corresponding image data is printed by another printhead 30.
Due to mechanical tolerances in the manufacturing process, it may be difficult to maintain an accurate alignment between the printheads 30 in a printing module 12. Moreover, even if the printheads 30 are perfectly aligned, differences in the aim of individual nozzles in the nozzle arrays 31 may make them appear to be misaligned in the printed image. Any such alignment errors can produce visible artifacts in the printed image.
Alignment errors between the printheads 30 in the cross-track direction can result in artifacts being produced at the boundaries between the printheads (e.g., dark streaks where the multiple nozzles print at the same location, or light streaks where no nozzles print at a particular location). Alignment errors between the printheads 30 in the in-track direction can result in artifacts being produced where portions of a linear feature in the image that spans the overlap region don't align with each other and appear to be broken.
U.S. Pat. No. 6,068,362 to Dunand et al., entitled “Continuous multicolor ink jet press and synchronization process for the press,” discloses a method for synchronizing printheads of a printing system. The printing system includes a plurality of printheads with optical sensors mounted “before” each printhead (upstream) at some predetermined distance. A print media passes beneath the printheads in order to permit the printheads to print marks thereon. The optical sensors capture an image of the marks which are input into a synchronization circuit. The synchronization circuit determines whether any deviation from the desired target is present. If there is a deviation, the synchronization circuit modifies the line spacing of the printhead of interest in order to compensate for the inaccuracies. In this system, the adjusted line spacings are based on an output of an encoder attached to the paper drive motor. Such a system requires extremely high cost encoders to provide the resolution needed for the registration demands of a printer system. It also is subject to errors associated with slip or coupling between the motor and the motion of the paper through the print zone. This system is also very susceptible to errors produced by variations in motor speed such as wow and flutter. In this configuration, there is an inherent time lag from image capture until the media passes beneath the printhead. This time lag in and of itself introduces another variable which is also subject to deviation from its desired target.
European patent document EP0729846B1 by Piatt et al., entitled “Printed reference image compensation system,” discloses a similar method for aligning the images for a plurality of different color channels in a multi-color printing system. Registration marks are printed in the margin of the image as the print media passes beneath each printhead. A camera positioned before a second printhead captures an image of the registration mark printed by a first printhead. This permits the second printhead to adjust its printing if a deviation in the expected position of the registration mark is detected from the captured image.
U.S. Pat. No. 7,118,188 to Vilanova et al., entitled “Hardcopy apparatus and method,” makes use of the redundancy of nozzles in the overlap region 34 to correct for cross-track alignment errors. Different masks are provided that use different nozzles in the overlap regions 34. In some embodiments, an appropriate mask can be selected by measuring the width of the band artifact produced in the overlap regions 34 for a printed image. In other embodiments, a test pattern is printed which includes different areas corresponding to a set of masks. The optimal mask is then selected by visual evaluation or automatic evaluation with an optical scanner for use in subsequent printing operations.
Commonly-assigned U.S. Pat. No. 8,104,861 to Saettel et al., entitled “Color to color registration target,” discloses a method for calibrating a multi-color inkjet printing system. A test target is printed that includes three marks printed with a first color in which two of the three marks are aligned along a first axis, and the third mark is offset by a predetermined distance along a second axis. The test target includes a fourth mark printed with a second color in which the intended position is aligned along the first axis with one of the first three marks, and is aligned along the second axis with another of the first three marks. The locations of the printed marks are detected and used to determine an appropriate alignment correction needed to align the first and second colors.
Commonly-assigned U.S. Pat. No. 8,123,326 to Saettel et al., entitled “Calibration system for multi-printhead ink systems,” which is incorporated herein by reference, discloses a calibration method to correct for alignment errors in an inkjet printer having multiple printheads. The method includes printing a first test mark using a first printhead and a second test mark using a second printhead. The nominal positions of the first and second marks are separated by a predetermined spacing in the cross-track direction, and are aligned in the in-track direction. An image capture device is used to determine the positions of the printed marks, and an error factor is determined based on the position of the second mark relative to the first mark. The pulse train used to control the second printhead is shifted responsive to the error factor to correct in-track alignment errors. One limitation of this method is that the necessary separation between the first test mark and the second test mark in the cross-track direction means that the in-track alignment of the printed image data will only be perfectly corrected at those cross-track positions. This does not ensure that the printed image data will be perfectly aligned at the boundaries between the printheads (e.g., at centerlines 36 in FIG. 2).
There remains a need for an improved method for aligning image data printed on a receiver medium using two printheads in a multi-printhead printer that overcomes the limitations of the prior art.